Spring steel has been widely used in the manufacture of springs, prominently in vehicles and industrial suspension applications. For use in a car suspension system, spring steel is required to exhibit high fatigue strength.
Recently, weight reduction and high power have been required for vehicles with the aim of reducing exhaust emissions and improving fuel efficiency. Accordingly, the design of coil springs useful for engines or car suspension systems has been directed toward improving stress resistance.
Particularly, coil springs for use in vehicle suspension systems are required to have excellent strength because these parts continuously endure loads. Further, corrosion resistance should be fully considered because they are exposed to an external environment.
Such coil springs for car suspension systems have been usually made of spring steel including mainly of carbon (C), silicon (Si), manganese (Mn), and chromium (Cr) and having a tensile strength of about 1900 Mpa. Moreover, research has been conducted into an inclusion control technique in which kinds and contents of alloy elements are controlled to improve fatigue life.
The foregoing is intended merely to aid in the understanding of the background of the present invention, and is not intended to mean that the present invention falls within the purview of the related art that is already known to those skilled in the art.